Many oil reservoirs have been discovered which contain vast quantities of oil, but little or no oil has been recovered from many of them because the oil present in the reservoir is so viscous that is is essentially immobile at reservoir conditions, and little or no petroleum flow will occur into a well drilled into the formation even if a natural or artificially induced pressure differential exists between the formation and the well. Some form of supplemental oil recovery process must be applied to these formations which decreases the viscosity of the oil sufficiently that it will flow or can be dispersed through the formation to a production well and therethrough to the surface of the earth. Thermal recovery techniques are quite suitable for viscous oil formations, and steam flooding is the most successful thermal oil recovery technique yet employed commercially.
Steam may be utilized for thermal stimulation for viscous oil production by means of a steam drive or steam throughput process, in which steam is injected into the formation on a more or less continuous basis by means of an injection well and oil is recovered from the formation from a spaced-apart production well. While this process is very effective with respect to the portion of the recovery zone between the injection well and production well through which the steam travels, poor vertical and horizontal conformance is often experienced in steam drive oil recovery processes. By vertical conformance, it is meant the portion of the vertical thickness of a formation through which the injected steam passes. A major cause of poor vertical conformance is caused by steam, being of lower density than other fluids present in the permeable formation, migrating to the upper portion of the oil formation to the remotely located production well. Once steam channeling has occurred in the upper portion of the formation, the permeability of the steam-swept zone is increased due to the desaturation or removal of oil from the portion of the formation through which steam has channeled. Thus subsequently-injected steam will migrate almost exclusively through the steam-swept channel and very little of the injected steam will move into the lower portions of the formation, and thus very little additional oil from the lower portion of the formation will be produced. While steam drive processes effectively reduce the oil saturation in the portions of the formation through which they travel by a significant amount, a large portion of the recovery zone between the injection and production systems is not contacted by steam and so a significant amount of oil remains in the formation after completion of the steam drive oil recovery process. The severity of the poor vertical conformance problem increases with the thickness of the oil formation and with the viscosity of the oil contained in the formation.
In view of the foregoing discussion, it can be appreciated that there is a substantial, unfulfilled need for a method of conducting a well-to-well steam injection oil recovery method in a manner which results in improved vertical conformance.